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Timing your fall nitrogen application

Q & A with Nutrien Ag Solutions

Figure 2: Impact of soil drainage on application of tall applied nitrogen.

Q: We’re often told fall applications of anhydrous ammonia should not be applied until soil temperatures are below 10 C. Why?

A: At one time fall NH3 applications were associated with Thanksgiving. With farm consolidation and the need to apply anhydrous on more acres in the fall, the question now is how early is too early? To understand the dynamics of this application we need to consider how nitrogen is stored in the soil.

NH3 is quickly converted to NH4+ (ammonium) once applied in the soil. NH4+ has a positive charge and is bound to clay particles and organic matter which protects it from losses such as leaching and denitrification. Under warm conditions NH4+ is converted to form NO3- (nitrate) with the aid of bacteria. It is this conversion that we are trying to reduce and prevent with fall applications as NO3- is susceptible to loss. Cooler soils and the application of nitrogen in a band both serve to reduce this microbial activity.

In deciding when to start fall NH3 applications, growers should consider drainage. Fields with good drainage are less prone to loss. Start applications on these fields. Fields with poor drainage may require the addition of an inhibitor. Poorly drained fields applied early can see a 20 per cent loss of nitrogen, as demonstrated in the chart at top.

In 2013, a study was completed looking at the effects of fall NH3 application timing. The work demonstrated little yield difference between early applications (mid-September) and late applications (mid-October) on well drained land.

Generally, by mid-September, well drained land has cooled enough to minimize losses of fall applied anhydrous and microbial activity slows significantly after soil temperatures reach approximately 1C. Protecting your nitrogen by applying it in a band two to four inches deep further reduces microbial activity and minimizes loss.

Twyla Jones, P. Ag, CCA, is a manager of agronomic services with Nutrien Ag Solutions in north central Alberta.

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